Use the link below and contact the seller with what you are looking for. I purchased a half set (one front and one rear) of OEM wheels a while back. He may be willing to break the set and sell you the wheel you need.
BMW Factory OEM 2006 19" M5 Wheels Brand New Set of 4 - eBay (item 230512468798 end time Jan-13-11 16:24:35 PST)
BMW Factory OEM 2006 19" M5 Wheels Brand New Set of 4 - eBay (item 230512468798 end time Jan-13-11 16:24:35 PST)